As wafers manufactured by a bonding method, there is a SOI wafer having a SOI layer (thin film) formed on a silicon oxide film being an insulator, for example. The SOI wafer has characteristics, such as small parasitic capacitance, high radioactivity resistance, and the like, since the SOI layer in a substrate surface portion to be used as a device fabricating area is electrically separated by the silicon oxide film (buried oxide film layer (BOX layer)) from the inside of the substrate. For that reason, the SOI wafer is expected to provide effects such as high speed and low-power consumption operation, prevention of soft-error, or the like, and thus it is considered as a promising substrate for high performance semiconductor device.
It is possible to manufacture the above-described bonded wafer by, for example, an ion implantation delamination method.
In this ion implantation delamination method (also called Smart-Cut (registered trademark) method), a base wafer and a bond wafer having a microbubble layer formed by implanting ions such as hydrogen ions or the like are bonded with each other (bonding step), the bonded wafer is delaminated along a microbubble layer as a boundary (delamination step), and a thin film is formed on the base wafer.
In this case, since there is polishing sag near the edge of the wafer, the bonding strength of the bonded wafer is reduced as closing to an outermost periphery. Hence, the thin film in an area around 2 mm from the edge is not usually transferred to the base wafer. As a result, in this boundary where transference of the thin film become discontinuous, the delamination is not caused in the microbubble layer but is caused by the thin film on the surface layer being broken (for example, refer to International Publication Pamphlet Wo01/027999).
The aspect will be shown in FIG. 5. After bonding a base wafer 1 with a bond wafer 2 having a microbubble layer 4 via an insulator 3 (FIG. 5(a)), the bonded wafer is delaminated along the microbubble layer 4 as a boundary (FIG. 5(b)). Thus, the bonded wafer is separated into a delaminated wafer 7 and a bonded wafer 8 having a thin film 5 formed thereon. As shown in FIG. 5(b), however, the delamination is caused by the thin film on the surface layer being broken (circled portions) in the boundary where transference of the thin film 5 becomes discontinuous on the way.
For this reason, surface geometry of a periphery 6 (commonly known as a terrace) of the thin film on the bonded wafer 8 does not result in controlled geometry, but it is indented like a ria, and furthermore, the periphery is dotted with many isolated islands. Cracks occur due to an effect of corrosion by performing oxidization and cleaning (including cleaning for removal an oxide film) from the periphery 6 of the thin film in such conditions during processes after the delamination, and breaks or chippings also occur, so that this leads to a cause of particle generation.
In order to solve such problems, there is commonly employed a method in which the periphery of the thin film is removed after the delamination step (thin-film periphery removal step). For example, there are described in International Publication Pamphlet WO01/027999 a method for masking an area except the periphery of the thin film to then remove the periphery of the thin film by wet etching, and a method for locally polishing only the periphery of the thin film.
However, since the method by above-described wet etching requires masking the thin film, a masking agent will directly contact with the surface of the thin film, so that there is a concern of generation of scratches or impurity contamination. There is a concern also in the above-described polishing method that a removal width cannot be reproducibly obtained well by an effect of irregular touch due to a temporal change in a polishing pad (buff). Moreover, these methods are not easy to perform because of the need to form the mask to the thin film.
Further, it is also required from a viewpoint of quality of the thin film not to degrade uniformity in film thickness of the thin film left after the removal upon removing the periphery of the thin film.